X-ray absorption spectra of small formic acid clusters, (HCOOH)(n), n = 1-4, were examined theoretically within the framework of density functional theory. For monomer, assignment of the first peak around 532 eV was consistent with the experiment, whereas the second peak around 535eV was assigned to a mixture of three bands, O1s(C=O)-> sigma*(OH), O1s(OH) -> pi*(OH), and O1s(OH) -> sigma*(OH) excitations. For the dimer, relative intensities of the oscillator strengths of O1s(C=O) and O1s(OH) -> sigma*(OH) excitations decrease due to strong hydrogen bond formation, whereas those of O1s(C=O) and O1s(OH) -> pi*(C=O) excitations are insensitive to the dimerization. (c) 2005 Elsevier B.V. All rights reserved.